Process for preparing zinc formaldehyde sulphoxylate



Patented May 12, 1942 rnocsss iron PREPARING ZINC FORM- ALDEHYDE SULPHOXYLATE Loren C. Hurd, Jenkintown, Pa., assignor to Riihm & Haas Company, Philadelphia, Pa.

No Drawing. Application March 17, 1941, SerialNo. 383,802

6 Claims.

The present invention relates to the manufacture of zinc formaldehyde sulphoxylate, and more particularly to the preparation of zinc formaldehyde sulphoxylate from basic zinc formaldehyde sulphoxylate.

Proposals have been made heretofore of methods whereby zinc formaldehyde sulphoxylate may be produced, but in general such methods have not met with success commercially since, in addition to zinc formaldehyde sulphoxylate, they simultaneously producesubstantial quantities of related products of little value, such as zinc formaldehyde bisulfite, a

A primary object of this invention is the production of zinc formaldehyde sulphoxylate by a process adapted to large scale operation without simultaneous formation of by-products that cannot be utilized. Another object is to maintain the zinc formaldehyde sulphoxylate stable during processing with consequent production of high yields of product. A further object is to produce a product of superior quality, particularly with regard to purity, stability, and crystal character.

In accordance with the present invention, advantage is taken of the reaction of basic zinc formaldehyde sulphoxylate with phosphoric'acid. Such reaction is as follows:

The reaction is carried out in the presence of water, and zinc phosphate which forms as a precipitate is separated from the zinc formaldehyde sulphoxylate which goes into solution. The solution of zinc formaldehyde sulphoxylate is concentrated and the salt is removed from the concentrated solution by crystallization.

The reaction is carried out at temperatures within the range of from about C. to about 100 C., and preferably at temperatures of from about C. to about 70 C., in the presence of excess formaldehyde added either in the form of an aqueous solution of formaldehyde or as paraformaldehyde. Amounts of excess formaldehyde of the order of from 1% to 5% based upon the amount of zinc formaldehyde sulphoxylate present or to be formed have been found to be particularly suitable, although amounts outside this ran e may also be used to advantage.

Removal of zinc phosphate from the solution of zinc formaldehyde sulphoxylate may be effected in any convenient type of filter, for example, in a filter press or filter box. Filtration may be carried out in a closed system so that free circulation of air is avoided, or the filtration may be conducted in the presence of an inert gas such as carbon dioxide.

A decomposition inhibitor is added to the zinc formaldehyde sulphoxylate solution, preferably immediately after removal of zinc phosphate from the solution. Such inhibitors aresubstances which tend to prevent (1) precipitation of zinc sulfite, (2) formation of formaldehyde trisulfide, and (3) decomposition of zinc formaldehyde sulphoxylate. Amounts of inhibitor of the order of from .5% to 4%, based upon the weight of zinc formaldehyde sulphoxylate present in the solution, have been found to be particularly suitable although amounts outside of this range have also been-found to be effective. As an inhibitor, there may be used a weak acid having a primary dissociation constant of from about 1x 10- to about IX 10*, such as formic or citric acid, or a mixture thereof. Another char acteristic of the weak acid is that it has a solustabilizers are glycerol, ethylene glycol, dipropylene glycol, trimethylene glycol, sorbitol, mannitol and other polyhydric alcohols of similar character. These stabilizers tend to lowerthe partial pressure of formaldehyde so that water is preferentially removed during the concentration. Another requisite of the stabilizer is that it be compatiblewith the weak acid used as an inhibitor. Glycerol has been found to be particularly suited for use as a stabilizer when formic acid is used as an inhibitor. The addition of stabilizer amounting to from about 2% to about 12% by weight of zinc formaldehyde sulphoxylate in the solution will produce the desired stabilizing effect. In most instances, approximately 5% of stabilizer has been found to be effective.

After removal of zinc phosphate from the zinc formaldehyde sulphoxylate solution preferably in the presence of excess formaldehyde, an inhibitor and a stabilizer are added and the solution is concentrated under reduced pressure and at a temperature below that at which zinc formaldehyde sulphoxylate will decompose, untila concentration, for example, of approximately 70% zinc formaldehyde sulphoxylate is reached. Mechanical losses of formaldehyde as well as losses due to retention of formaldehyde by the zinc phosphate are compensated for by theaddition of paraformaldehyde or an aqueous solution of formaldehyde prior to or during concentration. After concentrating the solution, the salt may be crystallized out either by continuous removal of water or by allowing the temperature to drop to about 55 C., or by adding a non-aqueous,

water-miscible precipitating agent, such as alcohol or acetone. The precipitating agent is added hot, e. g. at a temperature above 55 C., while the solution is stirred. Agitation is continued 1 for a few minutes after such addition, and the slurry is then filtered, whereupon the crystals are washed, for example, with benzol or with benzol and alcohol. The crystals are then rapidly dried at a temperature below that at which decomposition will take place, and preferably at temperatures of from about 75 C. to about 100 C., under reduced pressure.

The following is an illustrative example of a process within the scope of my invention. The parts referred to are parts by weight.

A slurry was formed. by adding 139 parts of basic zinc formaldehyde sulphoxylate to 235 parts of water and 33 parts of a 37% formaldehyde solution. There was slowly added to the slurry 44.5 parts of 85% phosphoric acid diluted with 25 parts of water, while the mixture was constantly agitated and maintained at a temperature of 40 C. Zinc phosphate which precipitated was removed from the solution containing zinc formaldehyde sulphoxylate, by filtration in an enclosed filter press so as to avoid free circulation of air with consequent oxidation of the sulphoxylate. The concentrated liquor remaining in the press cake was washed from the filter with water, and 3 parts of 50% formic acid solution and 5 parts of commercial glycerol were added. The solution was then placed in a vessel provided with a suitable agitator, one part of paraformaldehyde was added thereto, and concentration was effected under reduced pressure to 68-70% of sulphoxylate. At the end of the concentration the temperature reached about 72 C. The concentrate was then treated with warm denatured alcohol in an amount sufficient to precipitate the zinc formaldehyde sulphoxylate as crystals. Agitation was conducted during the addition of alcohol and. for about five minutes thereafter, whereupon the crystallized material was suctionfiltered to effect removal of alcohol. The crystals were then washed with benzol and dried in a vacuum oven at a temperature of from about may be made in the process of which the above example is illustrative. Such modifications as come within the scope of the appended claims are considered a part of this invention.

I claim:

1. The process for preparing zinc formaldehyde sulphoxylate which comprises reacting basic zinc formaldehyde sulphoxylate with phosphoric acid in an aqueous medium, removing zinc phosphate from zinc formaldehyde sulphoxylate dissolved in the aqueous medium, adding a decomposition inhibitor and a stabilizer to the solution of zinc formaldehyde sulphoxylate, concentrating the solution, precipitating zinc formaldehyde'sulphoxylate from the solution, and drying the precipitated zinc formaldehyde sulphoxylate.

2. The process for preparing zinc formaldehyde sulphoxylate which comprises reacting basic zinc formaldehyde sulphoxylate with phosphoric acid in an aqueous medium containing formaldehyde, removing zinc phosphate from zinc formaldehyde sulphoxylate dissolved in the aqueous medium, adding a weak acid, a zinc salt of which is water-soluble, and a polyhydric alcohol to the solution of zinc formaldehyde sulphoxylate, concentrating the solution, precipitating zinc formaldehyde sulphoxylate from the solution, and drying the precipitated zinc formaldehyde sulphoxylate.

3. The process for preparing zinc formaldehyde sulphoxylate which comprises reacting basic zinc formaldehyde sulphoxylate with phosphoric acid in an aqueous medium containing formaldehyde, removing zinc phosphate from zinc formaldehyde sulphoxylate dissolved in the aqueous medium,

adding formic acid, formaldehyde and glycerol to the solution of zinc formaldehyde sulphoxylate, concentrating the solution, precipitating zinc formaldehyde sulphoxylate from the solution anddrying the precipitated zinc formaldehyde sulphoxylate.

4. In a process wherein a solution of zinc formaldehyde sulphoxylate is concentrated, the improvement which comprises inhibiting oxidation and stabilizing the zinc formaldehyde sulphoxylate during concentration-by adding thereto a weak acid, a zinc salt of which is water-soluble, and a polyhydric alcohol.

5. In a process wherein a solution of zinc formaldehyde sulphoxylate is concentrated, the improvement which comprises inhibiting oxidation and stabilizing the zinc formaldehyde sulphoxylate during concentration by adding thereto formic acid and glycerol.

6. In a process wherein a solution of zinc formaldehyde sulphoxylate is concentrated, the improvement which comprises inhibiting oxidation and stabilizing the zinc formaldehyde sulphoxylate during concentration by adding thereto formic acid, formaldehyde and glycerol.

LOREN C. HURD. 

